Polyaxial bone anchors and methods of use in treating spinal disorders are known. Typical methods involve anchoring at least two screws or hooks into the vertebrae, and fixing the screws or hooks along a spinal rod to position or immobilize the vertebrae with respect to one another. The screws or hooks commonly have anchor heads with U-shaped channels in which the spinal rod is inserted and subsequently clamped by a fastener, such as, for example, a threaded nut, set screw, or locking cap. These methods commonly involve multiple screws or hooks and multiple spinal rods. The spinal rod(s) may be shaped to maintain the vertebrae in a desired orientation so as to correct the spinal disorder at hand (e.g., to straighten a spine having abnormal curvature). Additionally or alternatively, the screws or hooks may be spaced along the rods(s) to compress or distract adjacent vertebrae.
Surgeons may encounter difficulty with spinal fixation and stabilization methods because of difficulty aligning the spinal rod(s) with the U-shaped channels in the anchor heads of the screws or hooks. For example, the anchor heads are often out of alignment with one another because of the curvature of the spine or the size and shape of each vertebrae. To facilitate easier insertion of the spinal rods into the U-shaped channels, and to provide additional flexibility in the positioning of the spinal rods and the screws and hooks, bone anchors have been developed where the anchor member (e.g., screw or hook) and anchor head can initially pivot or rotate with respect to each other. These bone anchors are sometimes referred to as polyaxial bone anchors and the pivot or rotation of the anchor member is referred to as angulation.
A disadvantage of many polyaxial bone anchors is the degree to which the anchor head and member can angulate. Typical polyaxial bone anchors have anchor members that can rotate up to about 30° from a central axis extending down through the anchor head. It may be advantageous to provide polyaxial bone anchors with increased angulation.